In recent years, for the purpose of improvement of fuel efficiency etc., there has been known an internal combustion engine designed to control the intake air amount by cooperative control of a throttle valve and another mechanism. As such a mechanism, there is typically a variable valve operation mechanism for controlling the opening characteristics of an intake valve or exhaust valve, but there are also a variable cylinder number mechanism or variable exhaust volume mechanism etc.
Further, such an additional mechanism for controlling the intake air amount generally is set to a state in accordance with the intake air amount set in accordance with the accelerator opening degree etc. (that is, the target intake air amount) and the operating state of the internal combustion engine so that the fuel efficiency, emission, torque fluctuation, and other conditions become optimal, but an internal combustion engine itself is expected to operate (or generate torque) in accordance with the operation of the accelerator pedal by the driver, so it is necessary to realize the target intake air amount corresponding to the accelerator opening degree no matter what state the mechanism is set to. That is, it is necessary to find the opening degree of the throttle valve for realizing the target intake air amount corresponding to the setting of the mechanism (that is, the target throttle opening degree) and adjust the opening degree of the throttle valve to that opening degree.
As a method for finding the target opening degree of a throttle valve, for example Japanese Unexamined Patent Publication (Kokai) No. 2002-309993 discloses the method of successively calculating a required torque from the accelerator opening degree, a target intake air amount from the required torque, a target intake pressure from the target intake air amount, and a target opening degree of a throttle valve from the target intake pressure, by using an intake system model. However, what Japanese Unexamined Patent Publication (Kokai) No. 2002-309993 discloses is the case of using just a throttle valve to control the intake air amount. This does not correspond to a case of provision of an additional mechanism for controlling the intake air amount.
On the other hand, as a method for finding a target opening degree of a throttle valve able to deal with even the case of provision of such a mechanism, there is the method of creating in advance a map of the target opening degree using the target intake air amount and indicators of the operating state as arguments and finding it based on that map. However, in practice, preparing such a map requires tremendous time. In particular, when providing a mechanism such as explained above, indicators of its setting are added as indicators of the operating state, so the amount of the mapping work becomes extremely large. Further, if reducing the arguments or the number of measurement points so as to reduce the amount of the mapping work, the accuracy of the target opening degree obtained is liable to fall.